Ellipsometry is a sensitive optical technique for determining properties of surfaces and thin films. The shape and orientation of the reflected ellipse depend on the angle of incidence, the direction of polarization of the incident light, and the reflective properties of the surface being examined. The structural details of ellipsometers are more fully described in U.S. Pat. Nos. 6,449,043, 5,910,842 and 5,798,837, each of which is incorporated in this document by reference. Normal incidence ellipsometry is a widely used type of optical metrology, both for thin film measurements as well as scatterometry applications.
By including a rotating compensator that rotates about the propagating axis of the beam, certain limitations of rotating-polarizer or rotating-analyzer ellipsometers can be overcome. Such a configuration is commonly called a rotating compensator ellipsometer (RCE). The structural details of a prior art normal incidence rotating compensator ellipsometer are more fully described, e.g., in U.S. Pat. No. 7,173,700, which is incorporated herein by reference.
The prior art normal incidence rotating compensator ellipsometer describe in U.S. Pat. No. 7,173,700 includes an illumination source that produces a broadband probe beam. A polarizer is optically coupled to the probe beam to impart a known polarization state to the probe beam. The polarized probe beam is then optically coupled to a rotating compensator that is placed between the polarizer and a sample. The rotating compensator introduces a relative phase delay ξ (phase retardation) between a pair of mutually orthogonally polarized components of the probe beam. The rotating compensator includes a rotating optical component, such as a waveplate, that delays the light polarized parallel to its slow axis relative to light polarized parallel to its fast axis by an amount proportional to the refractive index difference along the two directions and the thickness of the plate, and inversely proportional to the wavelength of the light. After leaving the compensator, the probe beam is directed at normal incidence against the surface of the sample. The sample reflects or scatters the probe beam back through the compensator and the polarizer, which acts as an analyzer for the beam returned from the sample. A detector measures the intensity of the returned probe beam as a function of rotational angle of the compensator or analyzer. A processor analyzes an output of the detector to obtain the quantities related to the complex reflectances ra, rb of the sample. Such quantities include, e.g.: |ra|2, |rb|2, Re(rarb*), and Im(rarb*).
The complex reflectances ra, rb refer to the reflectance coefficients of an anisotropic sample, such as a grating. By way of example, one reflectance (e.g., ra) may be defined for light polarized parallel to the grooves of the grating. The other reflectance (e.g., rb) may be defined for light polarized perpendicular to the grooves. The complex reflectances ra, rb can also be used to define reflection coefficients of intrinsically anisotropic samples, in which case ra and rb correspond to reflectances for light polarized parallel to the two principal axes of the anisotropic material.
It is often advantageous to perform ellipsometry over a spectrum of wavelengths instead of a single wavelength source such as a laser Such a spectrum of wavelengths may be produced by a broadband light source, e.g. a Xenon and/or Oxygen arc lamp. All wavelengths are transmitted simultaneously through the system in a broadband probe beam and the different wavelength constituents returned from the sample may be separated in space after the polarizer by a dispersive element, such as a grating or a prism, and detected with an array detector such as a charge-coupled device (CCD) or a linear photo diode array (PDA). Such a broadband system, called a spectroscopic ellipsometer, offers the advantage of providing sample properties like the dielectric function of a material as a function of wavelength or, equivalently, energy. Further, spectroscopic ellipsometry is essential for samples with stratified single or multiple overlayers, which are encountered regularly in the manufacturing process of computer chips and memory devices. The penetration depth of light depends on the wavelength, so that the short wavelength part of the spectrum can be used to measure overlayer dielectric function as if it was bulk material, while the longer wavelengths penetrate deeper to reach the underlying interface, and together with knowledge of the dielectric function of the overlayer material provide the layer thickness. With thickness and dielectric function, the layer on top of the substrate may be comprehensively characterized.
Broadband operation is advantageous for many applications. Unfortunately, rotating compensator systems are not ideally suited for broadband operation. The difficulty encountered with RCE operation is a consequence of the fact that the retardation of the waveplate depends roughly inversely on the wavelength λ of light. However, for best sensitivity, the retardation is preferably that of a quarter wave over the entire spectral range.
Current single rotating waveplate designs typically employ a waveplate that works reasonably well over a wide spectral range, yet, due to the dispersive nature of the material out of which the waveplate is constructed (e.g., MgF2), the sensitivity is compromised at either the extremely short- or long wavelengths, or at both extremes. Specifically, with the retardation increasing towards the short wavelength end of the spectrum, the sensitivity of a rotating compensator ellipsometer gradually decreases and is reduced to that of an equivalent rotating polarizer system when it approaches 180°. Reducing the wavelength further, the sensitivity initially increases, assumes a second maximum at 270° but then hits a dead zone around 360° retardation, for which an RCE returns no phase information at all but becomes a simple off-axis reflectometer.
One could, in principle, circumvent wavelength restrictions of a conventional rotating-compensator system by constructing it with an achromatic compensator, such as a Fresnel rhomb. However, such achromatic compensator devices are non-trivial and expensive to manufacture, significantly bigger and heavier than standard waveplates, and generally feature unevenly distributed moments about the optical axis. Hence achromatic retarders are more difficult to use in a continuously, fast-rotating configuration than standard waveplates.
It is within this context that embodiments of the present invention arise.